The invention relates to connection-oriented packet-based telecommunications networks, such as ATM (Asynchronous Transfer Mode), and particularly to avoiding packet loss at handovers in such networks.
At present, there are two very interesting trends of development in the telecommunication, viz. mobile communication and broadband networks, in which bit rates typically exceed 2 Mbit/s. An example of broadband networks is Broadband Integrated Services Digital Network (B-ISDN), the transfer mode of which is selected to be Asynchronous Transfer Mode (ATM). The ATM is a switching and multiplexing solution particularly relating to a data link layer (i.e. OSI Layer 2, from here on called ATM layer). ATM enables an implementation of a connection-oriented packet network in the B-ISDN networks.
In ATM data transfer, the end user""s data traffic is carried from a source to a destination by means of virtual connections. Data is transferred over switches of the network in standard-size packets of 53 bytes, the packets being called ATM cells. The structure of an ATM cell is illustrated in FIG. 1. An ATM cell contains a header of 5 octets and an information field of 48 octets containing actual payload. The main object of the header is to identify a connection number for a sequence of cells, forming a virtual channel for a specific call. A physical layer (i.e. OSI Layer 1) may comprise several virtual paths, which are multiplexed in the ATM layer. The virtual paths are identified by means of a VPI (Virtual Path Identifier). Each virtual path may comprise a number of virtual channels, which are identified by a VCI (Virtual Channel Identifier). The header contains also other fields, such as an HEC (Header Error Control), a GFC (Generic Flow Control), a CLP (Cell Loss Priority) and a PT (Payload Type). The ATM cell contains indirectly an information on the receiver""s address, each cell thus being an independent data transfer unit. The number of cells transferred in a time unit is proportional to the user""s band-width requirements.
The ATM is a connection-oriented traffic technique, but because there is no connection before it is established, a connection establishment request shall be routed from a source through the ATM network to a destination approximately in the same way as packets are routed in packet-switched networks. After the connection has been established, the cells travel along the same virtual path during the connection.
A third trend of development is to introduce a wireless data transfer (wireless ATM) and mobility into the ATM networks (wireless ATM). The present B-ISDN and ATM standards do not support as such the additional features required by wireless communication, but different solutions to implement for instance mobility management in connection with wireless ATM have been presented already. The aim has been to add wireless data transfer and mobility to the ATM network without significant changes in the existing ATM standards and networks. Such solutions have been presented in the applicant""s copending Finnish Patent Applications 971178 and 970602, for example.
In PLMNs (Public Land Mobile Network), radio interfaces have conventionally been narrowband interfaces. The transmission systems of mobile networks have conventionally been implemented by circuit-switched connections in a star or tree network configuration. In order to increase the capacity and flexibility of the transmission systems, a use of different broadband packet-switched transfer systems or ATM technique in mobile networks has also been proposed, for instance in WO 9400959, EP 0366342 and EP 0426269. A possible future trend of development is mobile systems having a broadband radio interface. Then a broadband transmission system of the mobile system is also needed, while a potential alternative is ATM technique.
In mobile networks and in a wireless ATM network, a terminal does not have any fixed access point to the network, but the terminal and the access point may move in the network. When a virtual connection has been routed to the terminal through the ATM network, the routing must also be changed or extended from an old access point (a base station, for example) to a new one. This procedure is called handover or handoff. In a hard handover, the data transfer is interrupted, when the connection is connected from one access point to another. In a soft handover, the continuation of the data transfer is secured by the terminal having a connection both with the old and the new AP (Access Point) simultaneously during the handover.
In an ATM network, and generally in a packet-based telecommunications network, a loss of an ATM cell (packet) reduces significantly the efficiency of the connection. If one single ATM cell is lost (during handover, for instance), it may be necessary to retransmit a whole PDU (Protocol Data Unit) of higher level protocol, i.e. a plurality of ATM cells. This reduces the efficiency of the connection, and for this reason, cell losses have to be avoided. A change in cell order during a connection also causes an error situation at reception and starts a retransmission of the cells. Therefore, it should also be secured that the order of the cells does not change during handover. In general, this requires a buffering of cells always when an active connection breaks for whatever reason. Some reasons for cell buffering in a wireless ATM network are described below.
Traffic from an MT (Mobile Terminal) to an AP (uplink direction) and further to the network must be buffered at least in the MT, when the radio leg of the connection is broken. This may happen during a hard handover or a fading, for instance. A buffering of uplink traffic on the network side may be necessary during a path optimization process.
In downlink direction, there are several alternatives to implement cell buffering. The access point shall buffer the downlink cells for instance for sudden radio link interruptions, congestion and retransmission. During handover, the connection breaks approximately for the time needed for establishing a new radio connection and for releasing an old one. The buffering required by handover may typically be carried out either at an old access point AP or in an ATM switch. Buffering at the old access point AP is simple, because the AP needs a buffering of some kind in any case. Buffering relating to handover is a new function in the ATM switch, besides which, some of the cells still have to be buffered at the old AP.
In both above alternatives of downlink cell buffering, the old access point AP may comprise buffered cells, which have not yet been sent over the radio path. After a handover to a new access point AP, these unsent cells should be transmitted forward to the new AP without a loss of cells and maintaining the cell sequence in order to avoid a retransmission of a whole PDU.
As a solution to this problem has been presented that there are pre-established (permanent) connections between adjacent access points, over which connections the unsent cells can be transferred from the old AP to the new AP. This approach does not require any significant intelligence at the AP and does therefore not increase the complexity of the AP (base station, for instance). However, this solution is not very dynamic and reserves network resources only for this use.
As another solution to this problem has been presented an xe2x80x9cintelligentxe2x80x9d access point AP being able to establish a connection to a new AP by itself and to transfer the unsent cells to the new AP. By means of this solution, a better dynamism and utilization of network resources are achieved. However, a drawback is that the necessary additional intelligence increases the complexity of the AP and thus the costs.
The object of the invention is thus a method by which unsent cells can be transferred from an old access point to a new one without reserving network resources unnecessarily and without increasing the complexity of the access point and the price significantly.
The objects of the invention are achieved by means of a method of transferring unsent packets from an old access point to a new access point in a packet-based connection-oriented telecommunications network, in which the access point of a terminal to the network may change during an active connection, the method comprising the steps of: buffering at the old access point the packets which the old access point has not sent to the terminal before change of access point; establishing a connection from the old access point to the new access point; transferring the buffered packets from the old access point to the new access point over said connection. The method is according to the invention characterized in that said step of establishing a connection comprises the steps of: generating a signalling required for said connection establishment in a third network element, which is responsible for controlling the change of access point; tunnelling said signalling to the old access point; transmitting said signalling from the old access point to the new access point.
Another object of the invention is a packet-based connection-oriented communications network, comprising an arrangement for transferring unsent packets from an old access point to a new access point, in which arrangement the access point of a terminal to the network changes during an active connection, the arrangement comprising: means for buffering at the old access point the packets which the old access point has not sent to the terminal before change of access point; means for establishing a connection from the old access point to the new access point; means for transferring the buffered packets from the old access point to the new access point over said connection. The network is according to the invention characterized in that said means for establishing a connection comprise means for generating a signalling required for said connection establishment in a third network element, which is responsible for controlling the change of access point; means for tunnelling said signalling to the old access point; means for transmitting said signalling from the old access point to the new access point.
Still another object of the invention is a handover method for handing a terminal over from an old access point to a new access point during an active connection in a packet-based connection-oriented communications network, the method comprising the steps of: establishing a connection between the terminal and the new access point; buffering at the old access point the packets which the old access point has not sent to the terminal before the change of access point; routing an extension from the old access point to the new access point; transferring the buffered packets from the old access point to the new access point over said extension. The method is according to the invention characterized in that said step of routing the extension comprises the steps of: generating a signalling required for the routing in a third network element, which is responsible for controlling the change of access point; tunnelling said signalling to the old access point; transmitting said signalling from the old access point to the new access point.
In the invention, a connection from an old access point to a new one is established dynamically, when it is required. However, instead of being controlled by the old access point, the connection establishment is controlled by a third network element, which is responsible for controlling the change of access point anyway. This third network element is typically a switch or a controlling element of a mobile network, which element already comprises the xe2x80x9cintelligencexe2x80x9d required for connection establishment. The third network element generates a necessary signalling and in a way xe2x80x9ctunnelsxe2x80x9d this signalling to the old access point, which transmits the signalling forward without interpreting it. Thus the old access point can be made to send signalling messages and to establish a connection without needing any additional intelligence. The third network element controlling the change of access point knows the new access point and is thus able to address the signalling correctly. Tunnelling may comprise, for instance, an encapsulation of an original signalling message containing the address of the actual destination into a packet provided with the address of the old access point and sent to the old access point. The old access point decapsulates the signalling message from the packet and transmits the signalling message forward on the basis of the actual destination address. A signalling response possibly sent from the new access point is addressed to the third network element and transmitted automatically over the old access point. After the connection has been established, the old access point sends the buffered unsent packets to the new access point, in response to a command sent by the third network element, for instance. Then the third network element can release the connection in the same way as described above, if it is not needed any longer. In another embodiment of the invention, the connection establishment according to the invention is used for carrying out a PE (Path Extension) handover. The PE handover extends an existing connection from the old access point to the new access point on the principle described above. In the PE handover, the connection established is not released, however, after the buffered packets have been sent to the new access point.
By means of the invention, the complexity of the access point (a base station, for instance) can be kept minimal, which makes the price more cost effective. On the other hand, the intelligence required in the third network element is already available, and therefore, the invention requires only some new functionality there. A connection can be established dynamically, by which a permanent reservation of network resources is avoided.